I have 3 in prot for rage gen when changing stances. I've done all the lvl 70 instances as MT except BM and from time to time I OT in karahzan
Rhys, have your normal group get threat meters. They can help you hold aggro. Dps warriors need more help, because they don't have the improved threat gen from the prot tree. There are some things you can do to lessen the loss of aggro: Peircing Howl, Hamstring, intercep moves.. (The first 2 slow down mobs heading to kill your healers..and when it's safe, you can also fear)
Typically, not always..though, save all 3 of your taunts (yeah you have 3) for emergency snap aggro.
Taunt, Challenging Shout and Mocking blow. Use them wisely.. (You can use CS on a single targe.) If it's an emergency, blow it.
Other then that, find out which abilites produce the ..fastest, greatest threat and spam them.. I think revenge got nerfed..make sure any threat gen lists are up to date.
Typically in lvl 70 runs..in a good group, holding aggro perfectly is not required on trash mobs.. It's more important on bosses.. (This doesn't mean you shouldn't try as hard as you can, I'm just saying don't worry to much if you lose aggro now and again in a blitz style runs. It happens to prot warriors to.
Multi-tanking: Thunderclap, cleave, piercing howl.
Danny